1. Field of the Invention
The present invention relates to a stepless transmission, and more particularly to a stepless variable ratio transmission which is electronically controlled.
2. Description of Related Art
Heretofore, various devices have been disclosed which provide, in one manner or another, a transmission having a substantially continuous or "stepless" transmission ratio or gear speed change. Such transmissions suffer from one or more disadvantages, and one drawback in particular that is common to these transmissions is that they have relatively low efficiencies.
Those skilled in the art continue to strive to attain a design for a high-efficiency stepless transmission, particularly for vehicular applications. Examples of types of stepless transmissions propounded for use in automobiles or other vehicles include the expansion pulley type, the hydrostatic transmission, the hydrodynamic transmission, and tractional and frictional transmissions. The expansion pulley type, a contemporary example of which comprises hydraulically controlled expansion sheaves and a metal belt, has certain intrinsic disadvantages, primarily due to belt slippage. The other transmission types noted above all suffer from, in addition to low efficiency, either limited power and speed rates, bulkiness, noisy operation or excessive cost.
Planetary gear assemblies have heretofore been proposed for use in transmissions having a continuous or stepless variable transmission ratio. Several designs employing the general concept of using planetary gear assemblies in such stepless transmissions have been disclosed or depicted in the prior art.
A first group of these planetary gear stepless transmissions operate by splitting the power of the prime mover (motor) into two routes or two streams of power flow. This group of patents includes U.S. Pat. No. 4,109,550, to Murayama; U.S. Pat. No. 4,662,245, to Taylor; U.S. Pat. No. 3,427,899, to Gunderson; U.S. Pat. No. 3,404,584, to Trautman; U.S. Pat. No. 3,304,803, to Stockton; U.S. Pat. No. 3,303,725, to Knosp; U.S. Pat. No. 3,283,612, to Densham; and U.S. Pat. No. 3,224,298 to Hill. The operation of such transmissions generally involves varying a speed or movement in one of the two streams of power flow whereby an output terminal in the system obtains a variable ratio with respect to the prime mover. Such systems generally involve an even or nearly even distribution of power into the two streams of power flow. Thus, the variable ratio at the output terminal is obtained essentially controlling one stream of power flow with another of equal magnitude or on the same order of magnitude. Control systems using a control means which requires power comparable to the power flow to be controlled are generally considered to be of poor design, as approximately half of the available power is consumed in providing the variable ratio control.
A second group of planetary gear stepless transmissions operate by applying a braking force to a third terminal to change the transmission ratio between a first (input) terminal or prime mover and a second (output) terminal. Patented devices falling into this group include U.S. Pat. No. 3,220,284 to Horvath, and U.S. Pat. No. 3,529,493 to Egli.
A third kind of double differential system (described by Bargo, Popular Science, July, 1985) has been used to steer continuous track vehicles which controls power output to two different branches from thence the left side and right side tracks will obtain different speed and the control means is a self-locking worm gear.
This type of transmission generally performs adequately for applications where speed adjustment is only temporary, such as on continuous track vehicles, e.g., tanks or bulldozers. This type of variable ratio control has several disadvantages which substantially reduce its desirability in applications where the control is to be used for extended periods of time, namely excessive wear on moving parts, heat buildup, and high energy loss. These disadvantages result in very low operating efficiency and would lead to short service life, due to the basic operating principles employing energy dissipation from one power route to control the transmission ratio.
In the devices disclosed in all of the above-identified patents, the power needed to drive the variable speed mechanism or the braking mechanism is drained from the input prime motor. Sufficiently precise ratio control may thus not be attained in a vehicular transmission application, and the transmission ratio would be easily influenced by sudden changes of load, for example, when the vehicle drive wheels encounter stones or potholes on the road.
It is therefore a principal object of the present invention to provide high efficiency stepless variable ratio transmission having a relatively simple mechanical structure and being capable of operating at sufficiently high speed and high power to be used in vehicular applications.
It is a further object of the present invention to provide a high efficiency stepless variable ratio transmission having ratio control means especially suitable for use with electronic or computer control input.